a. Field of the Invention
The present invention relates to an electronically driven pressure boosting system that is used to boost the torque output of an internal combustion engine.
b. Related Art
One way to boost the torque and peak power provided by a reciprocating piston internal combustion engine, is to use a pressure boosting device to increase the mass airflow into the engine. The increased air supply then permits a greater amount of fuel to be combusted in each ignition event.
Examples of pressure boosting devices include turbochargers and superchargers, referred to herein collectively as xe2x80x9ccompressorsxe2x80x9d. A turbocharger is driven entirely or partly by energy in the exhaust stream. This is an efficient use of otherwise mostly wasted energy, but such devices suffer from the limitation that the boost is not available or significant at low engine speeds (rpms). The time taken for the boosted torque to become apparent to the driver is called xe2x80x9cturbo lagxe2x80x9d. Often, a driver may demand high torque from an engine at low rpms, for example at the start of an overtaking manoeuvre. If the pressure boost device is driven only by exhaust gasses, then boosted torque will not be available at low rpms.
One way of dealing with the limitation is to provide an electrical motor connected to the turbocharger, which is energised when the turbo boost is insufficient. This type of electrically driven pressure boosting device is, however, expensive in terms of hardware cost. Another solution is to use a supercharger, that is, a compressor device that is driven by means other than an exhaust gas turbine, for example via a mechanical linkage to the engine, or by an electrical motor driven from the vehicle battery and/or battery charging system. Mechanical supercharger systems can however, be mechanically bulky and expensive, and do not reduce xe2x80x9cturbo lagxe2x80x9d. Electrically driven supercharger systems provide a lower cost and compact solution, but can require a significant amount of electrical energy when driven, for example, up to three times the current which can normally be supplied by a typical motor vehicle 12 volt battery. A typical electrical motor for a supercharger driven from a conventional motor vehicle electrical power supply system, can take up to 0.5 seconds to reach operating speed. Although this is a considerable reduction in lag compared with an exhaust gas driven turbocharger, this is still a noticeable lag for the vehicle driver.
Motor vehicle alternators are typically specified to provide either all or most of the power requirement for the entire vehicle, the battery only being used to store sufficient electrical power to start the vehicle engine and occasionally deliver power when the accessory load exceeds the alternator output. Typical European vehicle alternators are specified to provide about 130 A of current, while an electrically powered supercharger can require in excess of 300 A. An alternator able to supply this much current is significantly more expensive, heavy and bulky than a conventional alternator.
Because the pressure-boosting device cannot be 100% efficient, there will also be inevitable electrical and mechanical losses associated with the device, that can place significant mechanical and thermal stress on components within the device.
The expense of increasing the capacity of the vehicle battery and charging system, or the dealing with inherent thermal and mechanical limits of components within the pressure boosting device, to meet any level of driver demand can easily outweigh the benefits of using an electrically driven compressor. Therefore it is important to drive such a device in an efficient manner, and within the limits of the vehicle electrical power supply, and thermal and mechanical limits of the device itself. At the same time, it is important to minimise lag and to maximise the torque boost benefit perceived by the driver over as wide a range of driving conditions as possible. Because the level at which an electrically driven pressure boosting device is driven, is essentially independent from the engine operating speed, it is therefore necessary to devise an appropriate control system for operating the pressure boosting device that takes account of the system""s limitations.
It is an object of the present invention to provide a convenient and economical electrical pressure boosting device and method for increasing the torque available from an internal combustion engine.
According to the invention, there is provided a torque boosting system for boosting the torque of an internal combustion engine, said system comprising means for generating one or more engine operating parameters, an electrically driven rotary compressor for assisting aspiration of the engine to boost engine torque, an electrical supply system for providing electrical power to drive the compressor, and an electronic control system responsive to the engine operating parameter(s) to control the operation of the engine and the compressor such that in an idle mode of operation of the compressor said device does not assist the aspiration of the engine, and in a boost mode of operation of the compressor said device does assist the aspiration of the engine, wherein:
a) the compressor in the idle mode of operation operates within a first range of speeds and in the boost mode of operation operates within a second range of speeds, the second range of speeds being greater than the first range of speeds;
b) when activated by the electronic control system, the compressor requires a lag time to accelerate from an idle speed within the first range of speeds to a boost speed within the second range of speeds;
c) the electronic control system monitors the engine operating parameter(s) and calculates therefrom a likelihood that the engine torque will need to be boosted by the compressor; and
d) when the compressor is operating at an idle speed, the electronic control system controls said idle speed so that the lag time varies inversely with the calculated likelihood that the compressor will be needed to boost engine torque.
Also according to the invention, there is provided a method of using a torque boosting system to boost the torque of an internal combustion engine, said system comprising means for generating one or more engine operating parameters, an electrically driven rotary compressor for assisting aspiration of the engine to boost engine torque, an electrical supply system for providing electrical power to drive the compressor, and an electronic control system responsive to the engine operating parameter(s) to control the operation of the engine and the compressor, wherein the method comprises the steps of:
i) operating the compressor in an idle mode of operation within a range of idle speeds in which the compressor does not assist engine aspiration;
ii) after step i), operating the compressor in a boost mode of operation within a range of boost speeds in which the compressor does assist engine operation, the compressor requiring a lag time in order to accelerate from an idle speed to a boost speed;
iii) prior to step ii), using the electronic control system to monitor one or more engine operating parameters, and to calculate therefrom a likelihood that the engine torque will need to be boosted by the compressor to meet future driver demand; and then
iv) using the electronic control system to vary the idle speeds so that the lag time varies inversely with the calculated likelihood that the compressor will be needed to boost engine torque.
The inverse relationship between the calculated likelihood and the lag time may be a simple 1/x inverse relationship, or any other suitable function in which the lag time decreases with increasing likelihood, or vice versa.
Several benefits flow from this approach. First of all, the higher the idle speed, the greater the reduction in lag time. The lag time is therefore reduced most in situations where it is judged most likely that compressor boost will be required.
Second, because the idle speed of the compressor is variable, the compressor will not in general be run continuously at a high idle speed. Since current consumption for commercially available compressor devices tends to increase with the square of the compressor speed, this results in a significant savings in terms of electrical power consumption. This in turn reduces the required capacity of the electrical supply system.
Third, a continuously high idle speed would result in the compressor becoming hot, owing to inevitable friction in moving parts, and less than 100% efficiency in the electric motor. By varying the idle speed according to the calculated likelihood of required compressor boost, the compressor temperature is kept lower than it would otherwise be at a continually high idle speed.
Fourth, because some rotational energy will be stored in the compressor prior to the acceleration of the compressor to boost speed, there will be a reduction in the peak electrical current needed to bring the compressor up to boost speed, for any given lag time.
The compressor may, of course, be kept idling for some time, but when the likelihood drops below a threshold value, the idling speed may be dropped to zero in order to conserve electrical power, and to reduce the temperature of the compressor.
In a preferred embodiment of the invention, the calculation of the likelihood that the engine torque will need to be boosted by the compressor to meet future driver demand includes a calculation using the history of one or more of the engine operating parameters. This calculation can then be weighted towards engine operating parameters from more recent times rather than less recent times. This helps to improve the accuracy of the likelihood calculation.
When the system is incorporated in a motor vehicle, the system may include a speed control system for generating a driver demand signal, for example an accelerator pedal and an electronic pedal position sensor, or an electronic cruise control system linked to an engine management system. One engine operating parameter can then be the driver demand signal. In a preferred embodiment of the invention, the electronic control system monitors pedal position and how xe2x80x9cbusyxe2x80x9d the pedal is, that is, the magnitude of pedal movement and the speed of such pedal movement, both towards higher driver demand and towards lower driver demand, as both types of movement are indicative of aggressive or passive driver behaviour.
The system may additionally include a gear change system through which the engine torque is transmitted, in which case one engine operating parameter is the state of the gear change system.
When the system includes an engine speed sensor, one engine operating parameter may be the engine speed. When engine torque required to satisfy the driver demand approaches the maximum torque available at a particular engine speed, then it becomes more likely that torque boost will be required.
In a preferred embodiment of the invention, there are a plurality of engine operating parameters, at least one of which is a limiting parameter that may restrict use of the compressor. The method then comprises the step of: using the electronic control system to monitor said limiting parameter(s), and to calculate therefrom a likelihood that said limiting parameters may limit the ability of the compressor to meet future driver demand; and then using the electronic control system to vary the idle speeds so that the lag time varies directly with the calculated likelihood that the ability of the compressor to meet future driver demand will be restricted.
One type of limiting parameter may be the temperature of the compressor. Therefore, the system may comprise a compressor temperature sensor, in order to measure the temperature of the compressor.
Since the system may include an electrical supply system for providing electrical power to drive the compressor, one limiting parameter may be the ability of the electrical supply system to provide electrical power to drive the compressor to boost engine torque. In a motor vehicle, the electrical supply system usually includes a battery and an engine-driven battery recharger. The method then comprises the step of isolating at least partially the battery from the engine-driven battery recharger to drive the compressor using the battery, said limiting parameter being the battery state-of-charge determined for example from the battery state-of-charge.
This takes advantage of the fact that in general the peak current available from the battery will be several times greater than the continuous current available from the battery charging device, e.g. an alternator. The battery charging device can then continue to provide a steady current to other electrical consumers, while the battery can at least for some time supply the relatively high current required to run the compressor, particularly when the compressor is an electrically powered supercharger.
When the electrical supply system includes a battery and an engine-driven battery recharger, said state of the electrical supply system can be determined from an electrical load on the battery recharger, for example as compared with a maximum acceptable load on the battery recharger.
Preferably, the calculation of said ability of the compressor to meet future driver demand includes a calculation using the history of one or more of the engine operating parameters. The calculation of the ability of the compressor to meet future driver demand may then be weighted towards said additional operating parameters from more recent times rather than less recent times.